1. Field of the Invention
The present invention relates generally to electronic countermeasures systems. More specifically, the present invention relates to an adaptive cross polarization jamming system for use on board an aircraft or the like which adapts to the polarization change of an incoming radar pulse from a monopulse radar.
2. Description of the Prior Art
Generally, monopulse radar is used on United States and foreign aircraft and missiles to track targets since monopulse radar is accurate and not as susceptible to electronic countermeasures (ECM) jamming when compared to other types of target scanning radar.
Monopulse radar from, for example, a missile tracking a target compares the phase front of an incoming radio frequency (RF) signal in four separate quadrants of an antenna aperture. The upper and lower quadrants are compared to determine the elevation of the target the missile is tracking, while the right and left quadrants are compared to determine the azimuth of the target. Range from the missile to the target is determined by the time it takes the transmitted pulse to return from the target to the monopulse radar. This allows the monopulse radar to be used to determine direction and range with one pulse (monopulse).
The comparisons of the quadrants of the antenna aperture are performed with sum and difference channels or magic-T""s within the monopulse radar system. A lobing pattern created from the resultant sum and difference channels enables the radar system to extract the angle of the RF signal returning from the target.
In the past, systems have been developed to jam enemy monopulse radar to increase the survivability of friendly aircraft, ships during a conflict. These systems are used to jam both enemy aircraft and the missiles the aircraft launch towards friendly aircraft. Without adequate and reliable jamming of the radar, aircraft are susceptible to tracking and intercept by an enemy""s weapons systems including their missiles and the aircraft that launch the missiles.
Cross polarization (X-pol) jamming systems are used to induce distortions in the sum and difference channels of the monopulse radar causing the radar to xe2x80x9cdrive offxe2x80x9d the real target. The radar reads an incoming RF signal which is returning from an angle which is not the true angle for the target""s present location. This type of jamming occurs when a large orthogonal (X-pol) signal from the target returns to the radar.
Non-adaptive cross polarization jamming systems operate on the principle that the target""s radar system is vertically polarized and that the target is flying straight in a level plane. A transmit feed on the jamming system may then be swept a few degrees about horizontal to ensure an orthogonal component is fed into the radar at some time during the sweep.
However, non-adaptive cross polarization radar jamming systems have certain disadvantages in that the jamming sweep is only about the horizontal. The jamming radar can then easily be defeated by changing the polarization of the tracking radar or incorporating a random roll into the tracking system. Rolling the tracking system off vertical causes the orthogonal component to be at an angle other than horizontal. The tracking radar cannot be jammed because it never receives an orthogonal signal.
Adaptive cross polarization jamming systems are configured to adapt to the polarization change of an incoming radar pulse signal. This is accomplished by using a receiver measuring set within the jamming system which measures the polarization of the incoming radar pulse signal. The transmit antenna polarization of the jamming system is then rotated to the orthogonal of the polarization of the incoming radar pulse signal to jam the radar of the tracking aircraft or target.
However, adaptive cross polarization jamming systems also have certain disadvantages in that the receiver measuring set utilized therein is very expensive to implement. The equipment needed to measure the polarization of the incoming signal is also bulky. In addition, adaptive cross polarization jamming systems, which are mechanical systems, can jam only one radar at a time since it can not simultaneously jam multiple polarizations with the same transmit antenna. The response time of a receiver measuring set type of jammer is also slow.
Accordingly, there is a need for a highly efficient, yet relatively simple in design jamming system which will effectively jam an incoming RF signal from a monopulse tracking radar which is tracking irregardless of the polarization of the incoming signal. There is also a need for a jamming system which is light weight, non-bulky and provides for relatively fast response times when jamming multiple monopulse radar systems.
The present invention overcomes some of the disadvantages of the prior art including those mentioned above in that it comprises a relatively simple and highly efficient adaptive cross polarization electronic countermeasures system which will effectively jam an incoming RF (radio frequency) signal from a monopulse radar tracking a target. The adaptive cross polarization electronic countermeasures system comprises a transmit antenna and a receive antenna which are identical antennas. The transmit antenna is rotated 180 degrees with respect to the receive antenna. The transmit and receive antennas are mounted facing the same direction allowing for reception of an incoming RF signal from a monopulse radar and transmission of the signal back to the monopulse once ECM jamming is applied.
The incoming RF signal is separated into vertical and horizontal components by coupling to vertical and horizontal feeds within the receive antenna. Each component is then amplified sequentially through the adaptive cross polarization electronic countermeasures system and transmitted back to the monopulse radar.
At the transmit antenna, the vertical component is now transmitted out of the horizontal feed of the transmit antenna 180 degrees out of phase with respect to the feed. The horizontal component is transmitted out of the vertical feed of the transmit antenna with no phase shift. This results in a transmitted electromagnetic field vector which is orthogonal to the input electromagnetic field vector.
Discrete vertical and horizontal components are being transmitted back to the monopulse radar with switching occurring every 500 microseconds. The vertical and horizontal components are summed within the monopulse radar producing the resultant orthogonal vector which provides cross polarization jamming.